Calcineurin is a calcium-dependent phosphatase that has emerged as an important signaling molecule in the kidney. Calcineurin is the target of drugs such as cyclosporin A (CsA), whose therapeutic use is limited by associated nephrotoxicity. Currently, calcineurin is known to function in signal transduction of key molecules including Agll, IGF-I and TGFbeta. However, action of calcineurin is cell-specific and there are at least two distinct models of calcineurin action in the kidney. First, we have shown that calcineurin is required for TGFbeta-mediated ECM accumulation in cultured mesangial cells (MCs) and inhibition of calcineurin protects glomeruli from ECM accumulation associated with type I diabetes in vivo. Conversely, CsA induces ECM accumulation in tubule epithelial cells (TECs) in vitro and in the tubulointerstitium in vivo. Furthermore, there is no significant protection from diabetes-induced ECM accumulation with calcineurin inhibition in the cortical tubulointerstitium. Understanding mechanisms of calcineurin signaling specificity in the kidney is key to targeting inhibition of calcineurin to prevent ECM accumulation in glomeruli and avoid CsA-mediated nephrotoxicity in the tubulointerstitium. Signaling mechanisms of calcineurin in renal cells are poorly understood and few targets of calcineurin phosphatase action have been described in the kidney. Our work has identified candidate pathways downstream of calcineurin that may be critical to cell-specific regulation of ECM proteins. Moreover, we have discovered that calcineurin A isoforms are differentially regulated in the diabetic kidney, suggesting that this may be a further level of cell-specific signaling in the kidney. Therefore, our hypothesis is the following: Cell-specific action of calcineurin in the kidney is the result of signaling specificity downstream of calcineurin, dephosphorylation of cell-specific targets, and/or action of different calcineurin isoforms. First, we will delineate downstream signaling pathways that may be involved in regulation of ECM accumulation in MCs and TECs. Next, we will identify cell-specific targets of calcineurin dephosphorylation. Finally, we will evaluate the specific contribution of calcineurin A alpha isoform and calcineurin A beta isoform to regulation of ECM accumulation in MCs and TECs. The goal of these experiments is to distinguish mechanisms of calcineurin-mediated regulation of ECM in glomeruli from calcineurin action that contributes to CsA-nephrotoxicity.